1.1 Field of the Invention
This invention relates to the field of well logging and, more particularly, to a well logging method and apparatus for evaluating formation properties, such as resistivity, in high-contrast thin-layer formations or at high dip angles, with greater accuracy than prior techniques. Still more particularly, this invention relates to an improved technique for acquiring multiple data during resistivity logging, processing these data, and creating a representation of resistivity or conductivity from a formulated difference of data from a particular depth and/or neighboring depths.
1.2 Description of Related Art
Resistivity logging is a well-known form of electromagnetic ("EM") propagation logging. [In the present application, any references to resistivity are intended to encompass its inverse, conductivity, and vice-versa.] Resistivity logging is used for locating and evaluating the properties of potential hydrocarbon bearing zones in subsurface formations. Porous formations having high resistivity generally indicate the presence of hydrocarbons, while low resistivity formations are generally water saturated.
Resistivity logging is realized in different ways. A well tool, comprising a number of transmitting and detecting devices for measuring various parameters, can be lowered into a borehole on the end of a cable, or wireline. The cable, which is attached to some sort of mobile processing center at the surface, is the means by which parameter data is sent up to the surface. With this type of wireline logging, it becomes possible to measure borehole and formation parameters as a function of depth, i.e., while the tool is being pulled uphole.
An alternative to wireline logging techniques is the collection of data on downhole conditions during the drilling process. By collecting and processing such information during the drilling process, the driller can modify or correct key steps of the operation to optimize performance. Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are known as measurement-while-drilling ("MWD") techniques. Similar techniques focusing more on measurement of formation parameters than on movement of the drilling assembly are know as logging-while-drilling ("LWD"). However, the terms MWD and LWD are often used interchangeably, and the use of either term in the present disclosure should be understood to include both the collection of formation and borehole information, as well as data on movement of the drilling assembly.
U.S. Pat. No. 3,551,797 describes a conventional EM propagation logging technique. The '797 patent describes the transmission of EM energy into the formations, where energy shed back into the borehole is measured by receivers to determine the relative attenuation and/or the phase shift of the EM energy propagating in the formation. See also B. Clark et al., Electromagnetic Propagation Logging While Drilling: Theory and Experiment, SPE SIXTY-THIRD ANNUAL TECHNICAL CONFERENCE AND EXHIBITION, paper 18117, 1988.
U.S. Pat. Nos. 4,968,940 and 5,594,343 (both assigned to the assignee of the present invention) disclose conventional well logging tools used to evaluate the resistivity of formations in LWD operations. The '940 patent concerns the determination of formation resistivity at different radial depths of investigation with the use of multiple receivers. The '343 patent concerns the determination of formation properties at different radial depths of investigation with the use of multiple transmitters.
Conventional propagation and induction techniques for evaluating the resistivity of formations have practical limitations. Neighboring layers in high-contrast (in terms of resistivity) thin-layer formations can corrupt the measurement results--known as shoulder effect. At high dip angles, horns and other artifacts are seen in the measured data. Modeling and actual measurements have confirmed these effects. See B. Anderson et al., Response of 2-MHz LWD Resistivity and Wireline Induction Tools in Dipping Beds and Laminated Formations, SPWLA THIRTY-FIRST ANNUAL LOGGING SYMPOSIUM, pp. 1-25, 1990. The cause of the horns is transverse magnetic ("TM") coupling which becomes important at high dip angles. The TM horns observed are useful to detect layer boundaries, but are detrimental for quantitative formation evaluation.
U.S. Pat. No. 5,184,079 describes a method for correcting data developed from a well tool, disposed at a dip angle in a well bore, to eliminate the effects of the dip angle on the measured data. Another method for correcting induction logs, at high apparent dip angles, was described by Barber et al., Interpretation of Multiarray Induction Logs in Invaded Formations at High Relative Dip Angles, SPWLA THIRTY-NINTH ANNUAL LOGGING SYMPOSIUM, June 1998. This method requires time-consuming model calculations, and cannot be applied at the well site while logging. An underlying criterion of the methods described by the '079 patent and Barber et al. is the requirement that the dip angle be known before the methods can be applied.
U.S. Pat. No. 5,508,616 describes an induction tool incorporating transmitter and receiver coils disposed in an inclined fashion along the tool axis. PCT Application WO 98/00733, Bear et al., describes a logging tool including triaxial transmitter and receiver coils. U.S. Pat. No. 4,319,191 describes a logging tool incorporating transversely aligned transmitter and receiver coils. The techniques proposed by these disclosures do not address the evaluation of high-contrast thin-layer formations at high dip angles. U.S. Pat. No. 5,115,198 describes a method and apparatus for measuring the dip and strike of formations utilizing a tool with a triaxial receiver coil. U.S. Pat. No. 5,757,191 describes a method and system for detecting formation properties with a tool including triaxial transmitter and receiver coils. However, the proposed techniques require that the dip and/or strike angle be ascertained in order to practice the methods.
It is desirable to obtain a simplified method and apparatus for accurately evaluating the resistivity of formations with neighboring layers of high-contrast thin-layer formations or at high dip angles. Still further, it is desired to implement a logging technique that is not limited to piecewise constant model formations or prior knowledge of dip angles. Thus, there remains a need for a simplified logging process and apparatus to produce accurate resistivity profiles in these situations.